Semiconductor light-emitting diodes (“LEDs”) are used as light sources in a number of different applications. For example, LEDs are currently employed as light sources in flat-panel displays, fiber-optic systems, and many other applications because they are relatively inexpensive, small, and rugged. FIG. 1 shows one currently available design for an LED 100. The LED 100 includes a p-n semiconductor structure 102 having an n-region 104, a p-region 106, and a depletion region 108 (also known as an active region) formed between the n-region 104 and p-region 106. An electrical contact 110 is formed on the n-region 104 and an electrical contact 112 is formed on the p-region 106. Although, in many designs, the locations of the n-region 104 and p-region 106 may be switched.
As shown in the energy band diagram 200 of FIG. 2, forward biasing the LED 100 by applying a voltage across the electrical contacts 110 and 112 promotes electrons 202 from the conduction band of the n-region 104 and holes 204 from the valence band of the p-region 106 into the depletion region 108. Under forward bias, the valence band electrons 202 can recombine with the conduction band holes 204 in the depletion region 108. In direct band gap materials (e.g., gallium arsenide or gallium nitride), the recombination of a conduction band electron 202 with valence band hole 204 is a radiative energy transition in which electromagnetic radiation hv is emitted with energy equal to the energy band gap Eg of the p-n semiconductor structure 102. As shown in FIG. 1, a portion of electromagnetic radiation generated due to the electron-hole recombination in the depletion region 108 is transmitted through the p-region 106 into free space represented as electromagnetic radiation 114. The electromagnetic radiation 114 emitted from the depletion region 108 is generally incoherent and over a wide angular range. Collimation of the electromagnetic radiation 114 is desirable in certain applications, such as backside illumination in certain types of displays. Typically, collimation of the transmitted electromagnetic radiation 114 from the LED 100 is achieved using external optical elements, such as lenses, which increase the size and complexity of the LED 100.
In addition to lack of collimation, only a portion of the electromagnetic radiation generated in the depletion region 108 is transmitted through the p-region 104 as the electromagnetic radiation 114. A large portion of the electromagnetic radiation generated in the depletion region 108 remains confined within the p-n semiconductor structure 102 due to total internal reflection of the generated electromagnetic radiation at interfaces 116 and 118 between the relatively high refractive index p-n semiconductor structure 102 and the relatively low refractive index surrounding medium. Due to confinement of a large portion of the emitted electromagnetic radiation, typically, only about five percent of the electromagnetic radiation generated in the depletion region 108 is output from the LED 100 as the electromagnetic radiation 114.
One approach for improving the extraction efficiency of a light-emitting device is to form a two-dimensional photonic grating adjacent to an active region of the light-emitting device. For example, a two-dimensional photonic grating formed in a cladding layer of a quantum-well structure has been used to purportedly improve extraction efficiency of electromagnetic radiation emitted from an active region of the quantum-well structure. However, the two-dimensional photonic grating is located in such close proximity to the active region that the probability of non-radiative recombination at the interface between the two-dimensional photonic grating and the quantum well is undesirably high. Another approach for purportedly improving extraction efficiency of a light-emitting device is to place the light-emitting device within a two-dimensional photonic grating and design the photonic grating so that the frequency of light emitted from the light-emitting device falls within the photonic band gap of the photonic grating. Accordingly, designers, manufacturers, and users of light-emitting devices continue to seek improved light-emitting devices having an improved output efficiency and collimation characteristics for use in a multitude of applications.